Such fiber-optic amplifiers are especially used in optical telecommunication systems. They are inserted into optical transmission lines and amplify the signal light conducted in the transmission line. To control the light output, it is desirable to measure the input or the output of the amplifier, for example.
Fiber-optic amplifiers are known from two articles in the "Journal of LIGHTWAVE TECHNOLOGY", February 1991, Vol. 9 no. 2, pages 261 to 265 and pages 291 to 296, in which the signal or pump output is detected by a branch on the output side of the pump coupler. According to the pump coupler being used, it can be polarization- or wavelength-dependent. However, only one detector can be located on the monitor arm of the pump coupler.
From the publication "Optical Amplifiers and Their Applications, 1992 Technical Digest (Optical Society of America, Washington, DC, 1992), Vol. 17, pages ThA4-1/91 to ThA4-4/94", it is known to splice separate measuring couplers to the input and/or output of the fiber-optic amplifier, and to detect the light output coupled out in one branch. This is due to the requirement placed on the optical transmission systems, that the transmission line must be optically safe, i.e. at no time can the eye of a person who intentionally or unintentionally looks into a disconnected optical fiber, be in danger. To ensure this, the system, or individual system components, must be shut off in the event of a fiber breakage.
An interruption of the transmission line can be detected by monitoring the input light level of a fiber-optic amplifier, for example because the lack of input light can mean that the transmission line has been interrupted upstream of this fiber-optic amplifier.
From the above cited literature it is known to monitor the input light level of a fiber-optic amplifier. There, a portion of the input light level is coupled out by a coupler, which is part of the fiber-optic amplifier, and detected by a photodiode. The signal-to-noise ratio is degraded, since the amplifying part of the fiber-optic amplifier has less light output available, and the insertion of the coupler causes additional attenuation. Furthermore, this coupler creates additional material and labor costs.